Key switches having illuminating functions are often used in operating panels mounted to various electronic apparatuses, such as portable telephones and portable data terminals. Such illuminating functions are performed by directly illuminating keys with a light-emitting diode (LED) or by transmitting light from an LED through a light guide sheet.
FIG. 4 is a sectional view of switch 501 disclosed in Japanese Laid-Open Patent Publication No. 2007-53063. Central fixed contact 101 and peripheral fixed contact 102 are provided on an upper surface of circuit board 100. Sheet 110 is attached to the upper surface of circuit board 100 with adhesive layer 111. Sheet 110 has dome portion 120 protruding upwardly. Movable contact 130 having a dome shape is accommodated in dome portion 120. A lower surface of the central portion of movable contact 130 faces central fixed contact 101 with a gap between contacts 101 and 130. An outer edge of movable contact 130 is mounted onto peripheral fixed contact 102. An upper surface of movable contact 130 is held on dome portion 120 of sheet 110 with adhesive layer 111.
Sheet 110 is a light guide sheet that transmits light incident from a side edge surface, and illuminates the upper surface.
Light source 150 is mounted on circuit board 100. Light source 150 includes light-emitting portion 151 facing the side edge surface of sheet 110 to cause light from light source 150 to enter in parallel with sheet 110 made of a light guide sheet. Transparent resin 160 collects light from light-emitting portion 151, and introduces the light efficiently onto the side edge surface of sheet 110.
An operation of conventional switch 501 will be described below.
As a pressing force is applied to movable contact 130 from above dome portion 120 of sheet 110, the force reverses the dome shape of movable contact 130 when the force exceeds a predetermined level. Then, movable contact 130 contacts central fixed contact 101, thereby connecting central fixed contact 101 electrically with peripheral fixed contact 102. And, when the force is released, movable contact 130 returns to have its original dome shape shown in FIG. 4, thereby electrically disconnecting central fixed contact 101 from peripheral fixed contact 102.
When light source 150 emits light, the light enters through the side edge surface of sheet 110, the light guide sheet. The light is introduced into sheet 110 including dome portion 120, and illuminates the upper surface of sheet 110. Dome portion 120 covers movable contact 130 along the upper surface of movable contact 130. A key made of light-transmissible material and having a mark, such as characters or symbols, may be provided on sheet 110. This structure provides switch 501 which is operated easily even in the dark, in which the key is activated, and the mark is visibly recognized.
In switch 501, sheet 110, the light guide sheet, has a shape along the dome shape of movable contact 130. The light guide sheet having the light entering the side edge surface thereof may preferably thick so as to accept the light easily. For example, the sheet may often have a thickness of about 0.2 mm. However, if sheet 110, the light guide sheet, has the thickness of 0.2 mm, an excessive force is required due to a stress of sheet 110 when depressing movable contact 130, accordingly preventing switch 501 from being activated easily.